1. Field of the Invention
The present invention relates to an antenna for use in a radio communication apparatus such as a mobile communication apparatus and a radio communication apparatus including the antenna.
2. Description of the Related Art
From the viewpoint that miniaturization and frequency adjustment can be easily achieved, surface-mount antennas are often used for radio communication apparatuses such as terminal units (mobile telephones) for use in a mobile telephone system. In such a general surface-mount antenna in the related art, a radiation electrode is provided on a surface of a dielectric substrate to form an inductor, and an open end of the radiation electrode is spaced from a feed electrode to form a capacitor. Thus, an LC resonance circuit is provided.
In recent years, as disclosed in Japanese Unexamined Patent Application Publication No. 2005-318336, in accordance with the increase in the number of functions of mobile communication apparatuses such as mobile telephones, surface-mount antennas with improved antenna efficiency and a wider bandwidth which are capable of performing multiband communication have been proposed.
FIG. 1 is a perspective view illustrating a configuration of an antenna disclosed in Japanese Unexamined Patent Application Publication No. 2005-318336. An antenna 1 is disposed in a corner of a mount board 201 of a radio communication apparatus such as a mobile telephone. In a non-ground region 201a (a region where a ground electrode 201b is not formed) in the corner of the mount board 201, a parallel radiation electrode pattern 3 and a surface-mount antenna component 4 are provided. Using the parallel radiation electrode pattern 3 and the surface-mount antenna component 4, a parallel resonance circuit 2 is formed in the non-ground region 201a. A high-frequency current is supplied from a feeding point 5 to the parallel resonance circuit 2.
The parallel resonance circuit 2 is obtained by connecting the surface-mount antenna component 4 in parallel to the parallel radiation electrode pattern 3 formed in the non-ground region 201a. The parallel radiation electrode pattern 3 is provided in the form of a loop to occupy most of the non-ground region 201a and is open at a bottom of the surface mount antenna component 4. Thus, the parallel radiation electrode pattern 3 of the parallel resonance circuit 2 forms an inductor L. The inductance of the inductor L can be adjusted in accordance with the length of the parallel radiation electrode pattern 3. The surface-mount antenna component 4 is connected to the parallel radiation electrode pattern 3.
The surface mount antenna component 4 includes a pair of electrodes 41 and 42. The electrodes 41 and 42 are provided on a surface of a rectangular parallelepiped dielectric substrate. A capacitor Cd corresponding to a distance d is formed.
However, in the case of the antenna in the related art illustrated in FIG. 1 in which the surface-mount antenna component functioning as a part of the inductor and the capacitor is connected to the loop radiation electrode pattern formed in the non-ground region of the mount board, it is impossible to set a resonance frequency of the antenna to a desired low value because an area required for the parallel resonance circuit is large.
Accordingly, it is necessary to set an inductance value of the inductor L1, which affects the resonance frequency of the antenna in the matching circuit composed of the inductors L0 and L1 illustrated in FIG. 1, to a large value. As a result, a larger loss occurs in the matching circuit. If the non-ground region 201a becomes larger and the path length of the parallel radiation electrode pattern 3 becomes longer, an antenna having a desired low resonance frequency can be implemented. However, this leads to the increase in the size of the antenna.